Hydrocracking is a process for converting of heavy oil fractions to light oil fractions. Hydroprocessing is taking place in a hydroprocessing catalytic reactor which is the key element of a hydroprocessing unit. Hydroprocessing catalytic reactors can have single or multiple catalyst beds. Which of the options will be chosen for a particular reactor depends on the amount of catalyst is required for conversion of the feed to the product with desired properties. Most of the hydroprocessing reactions are exothermic and heat is developed as feed is passing through the catalyst bed. In order not to expose the catalyst to higher temperatures than required, and consequently to accelerate deactivation of the catalyst, the required volume of catalyst is divided into a number of beds with cooling zones (quench sections) installed between the beds. The cooling is achieved by introduction of cold hydrogen gas through the “quench pipe”. Besides the cooling, the quench zone must achieve spatial uniformity of species/temperatures of the liquid phase leaving the section to the lover bed. For this purpose a mixing chamber is installed in the section.
After the cooling and mixing stage the fluid must be distributed evenly on the catalyst of the bed below. For this purpose distribution trays are installed below the mixing chamber and above the catalyst of the lower bed. In order to achieve the highest distribution quality it must be assured that the distributor trays operates within own sensitivity limits. Distributor trays can operate as required as long as liquid depth on the tray is no more than 10 mm different from end to end of the tray Therefore, flow parameters of the fluid exiting the mixing chamber are often changed by different means and brought to the values required for the best performance of the distributor tray.
As it can be seen from the descriptions above the current organisation of the quench section consists from: quench pipe, mixing chamber, splash plate, rough cut tray (optional) and distributor tray. In some approaches the catalyst support grid, holding the catalyst of the higher bed, as well as the catalyst grid support beams are considered as a part of the quench section.
These elements are occupying lot of reactor space by own volume as well as free inter-element volume required to provide access to each of the elements for purpose of installation, un-installation, maintenance and cleaning of the elements.
The height of the quench section is the distance from the catalyst of lower bed to the catalyst of the bed above. The volume occupied by quench section is “inactive” reactor volume and decreasing this volume is paramount if increase of the active reactor volume is to be achieved. The space saved by decrease of the quench section height can be used for loading of the additional catalyst (revamp) or reduction of total reactor height (new reactors).
Known art mixers propose a solution to the problem of effective mixing and space requirements for the mixer. U.S. Pat. No. 8,017,095 discloses means to provide mixing of gas and fluids in a height constrained interbed space of a catalytic reactor without increasing pressure drop. In particular, the device improves the effectiveness of an existing mixing volume in mixing the gas phase and liquid phase of two-phase systems. According to U.S. Pat. No. 8,017,095, the mixing device helps create a highly arcuate flow to incoming effluents and a high degree of mixing within a constrained interbed space of a catalytic reactor.
Due to the installation, un-installation and maintenance requirements as well as for cleaning of all elements of the quench section it is crucial to provide enough space for full access to any of the elements. In order to minimise complexity of the work in the reactor it is necessary to enable convenient and fast opening/closing of all man-ways of all of the elements of the section.
Therefore a need exists to provide an improved efficient mixer which occupies less reactor space, especially vertically, than known mixers and which mixes effectively with a limited pressure drop.